<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
<style>
    div{
        width: 200px;
        height: 50px;
        background: yellow;
        border: 1px solid gray;
    }
    div ul{
        height: 0;
        margin: top 50px;
        overflow: hidden;
        transition:all 1s;
         
    }
    div:hover ul{
        height: 200px;
    }
</style>
</head>
<body>
    <div>
        我的菜单
        <ul>
            <li>111</li>
            <li>222</li>
            <li>333</li>
            <li>444</li>
            <li>555</li>
        </ul>
    </div>
</body>
</html>